1. Field of Invention
The invention pertains to an accelerator system, such as an accelerator system particularly suited for synthetic polyisoprene latex. The accelerator system comprises dithiocarbamate and thiourea. The accelerator system can, for example, produce synthetic polyisoprene films having a tensile strength of about 3,000 pounds per square inch (“psi”) to about 5,000 psi at low curing temperatures.
2. The Prior Art
Synthetic polyisoprene latex is chemically very similar to natural rubber latex. However, the physical properties of the ‘vulcanized’ films have never equaled those of the natural product. Natural latex films can be cured quickly (about 15 to 20 minutes) at low temperatures (about 90 to 100° C.) with a variety of accelerators to produce tensile strengths of about 4,000 psi to 5,000 psi (about 27 to 35 MPa). Polyisoprene latex films having high tensile strengths prior to the invention described herein, were very difficult to cure, primarily because no suitable accelerator system was available to promote the sulfur crosslinking. Regardless of the accelerator system selected for curing, only tensile strengths of about 500 psi to 2,000 psi (3 to 10 MPa) were achieved.
U.S. patent application Ser. No. 2002/0173563A1 describes a process for making dipped articles from latex involving the use of an accelerator system comprising zinc diethyldithiocarbamate (“ZDEC”), zinc 2-mercaptobenzothiazole (“ZMBT”), and diphenyl guanidine (“DPG”). ZDEC with the ZMBT is a popular accelerator system for natural rubber latex. Only the addition of DPG allows this accelerator system to achieve cured films with tensile strengths in excess of 3,000 psi (20 MPa).
U.S. Pat. No. 6,618,861 concerns a polyisoprene latex compound that includes an accelerator system of 2.0 parts per hundred (“phr”) tetramethylthiuram disulfide (“TMTD”), 0.2 phr zinc 2-mercaptobenzothiazole (“ZMBT”), 0.2 phr zinc dibutyldithiocarbamate (“ZDBC”), 0.2 phr 1,3-diphenyl-2-thiourea and 0.2 phr zinc diethyldithiocarbamate (“ZDEC”). However, after curing, this accelerator system provides tensile strength only of about 1,900 psi.
Thiourea (thiocabanilide) and thiourea/guanidine accelerator systems are known for advancing the cures of polychloroprene latex compounds. Sodium dithiocarbamate/thiuram accelerators are also recommended for the same purpose. (J. Carl, Neoprene Latex, E. I. duPont deNemours & Co., Inc., 1962 and J. Fitch, Polychloroprene Latexes, The Vanderbilt Latex Handbook, Third Edition, R. T. Vanderbilt Company, Inc., 1987). However, these accelerator systems are recommended to advance the bisalkylation crosslinks of zinc chloride initiated by the metal oxides. They do not presume to significantly affect sulfur crosslinking as in polyisoprene latex where no allylic chlorine atoms are present.